In a known process for manufacturing a multi-layer ceramic substrate so as to have metallic features including wire bonding pads and so as to have a cavity for a microelectronic chip, an inorganic filler-polymer composite slurry is prepared by dispersing inorganic particulates, typically alumina and silica, and organic vehicles, which include polymeric binders, such as polyvinylbutyral, plasticizers, and suitable solvents. After being ball milled, the resultant slurry is tape cast on a continuous casting machine to form green (unfired) sheets.
After being dried to remove the solvents, the sheets are blanked to the appropriate size. After blanking, the sheets are punched to form vias, each approximately five mils in diameter, whereupon lasers are used to cut individual cavity apertures in some or all of the sheets. Alternatively, individual cavity apertures are punched in some or all of the sheets when the sheets are punched to form vias.
Thereupon, the sheets are screened with a metal-polymer composite paste, which forms metallic features including electrical lines and wire bonding pads on the sheets and which fills the vias. The metal-polymer composite paste is prepared from a metallic powder and organic vehicles including a polymeric binder, a surfactant, and a solvent. An example of the metal-polymer composite paste is molybdenum-filled ethylcellulose.
After screening, the metallized sheets are dried in a forced-air oven. After drying, the sheets are stacked on one another in a suitable order so that the apertures in the stacked sheets define the cavity generally, and a metallic insert is inserted into each of the generally defined cavities in the stacked sheets. The metallic insert may be also called a metallic plug. After the metallic insert is inserted into each of the generally defined cavities, the stacked sheets are laminated at a sufficiently high pressure and at a sufficiently high temperature, so that interlayer bonding occurs due to softening of the binder, to form a green (unfired) laminate and to remove the volatile solvent. After the green laminate is formed, the metallic insert in each cavity is removed, and the green laminate is diced into individual substrates, which are sintered. In subsequent steps, microelectronic chips are mounted in the cavities in the individual substrates, and wire bonding techniques are used to bond electrodes on the chips electrically to wire bonding pads in the cavities. The individual substrates are known also as cavity substrates.
As a practical necessity, because a metallic insert is an expensive, precisely made element, it must be reused. A metallic insert must be cleaned thoroughly to remove any ceramic green sheet and metal-polymer paste residues, so that it can be reused. It is important that the cleaning process assures complete removal of any adhering residues before reuse, for effective manufacturing. It also is important that the cleaning process maintains the surface finish of a metallic insert, which may have a protective coating, when the metallic insert is cleaned.
According to this invention, an effective method is provided for removing ceramic green sheet and metal-polymer paste residues from a metallic insert while maintaining the surface finish of the metallic insert.